[python-frontend] Operational model for int.from_bytes()

[brcfarias]

• Adding support for int.from_bytes

>> int.from_bytes(b'\x00\x10', byteorder='big')
16

• Given that the frontend still doesn't support strings, the AST is being manipulated to replace the "byteorder" parameter with a boolean "is_bigendian".
• "bytes" array data is being encoded in base64 because json.dump() adds escape characters for non-printable ASCII characters. Additionally, in some cases it was not possible to diferentiate text from numerical values (e.g. "AAFF"). Now, "bytes" data is encoded in base64, and we store the corresponding int value after decoding in python_converter.
• Adding new "consensus" type mappings.

[lucasccordeiro]

Thanks for submitting this PR, @brcfarias.